This invention relates to a platform cooling arrangement for the nozzle guide vane stator of a gas turbine arranged downstream of the combustion chamber, with cooling-air ejection ducts passing through the wall of the combustion chamber, the wall of the platforms and/or the wall of a spacer located between the combustion chamber and the platforms, these cooling-air ejection ducts being arranged on the circumference of the respective wall, in at least one continuous or discontinuous row or in any pattern, to feed cooling air taken from the compressor of the gas turbine to the main gas flow surfaces of the platforms for film cooling.
The above type of cooling of the platforms of the nozzle guide vanes arranged downstream of the annular gas exit opening of the combustion chamber of a gas turbine and forming a stator assembly confined by the inner and outer platforms is known from Specification DE 198 13 779 A1, for example. Here, cooling air taken from the compressor is blown into the boundary layer of the hot-gas flow via cooling-air holes provided in the combustion chamber wall in the area of the exit opening or also directly in the platforms or a spacer between the combustion chamber and the platforms. By blowing in cooling air, the temperature of the hot-gas flow discharged from the combustion chamber is reduced in a flow layer contacting the inner surfaces of the platforms in order to shield the platform material from the remaining, uncooled hot-gas flow. If left unprotected, the platform material would be subject to so high a thermal load that the life of the platforms of the nozzle guide vanes would be significantly reduced. However, the cooling-air ejection holes, which usually are circumferentially distributed in the area of the annular exit opening of the combustion chamber or near the leading edge of the annularly arranged platforms, respectively, are not capable of effectively shielding or cooling the entire inner surface of the platforms against the hot-gas flow, this being due to the complicated flow conditions in the wall-near area, and also to the interaction between the hot-gas flow and the blown-in cooling air. This is attributable to a three-dimensional inlet boundary layer separation along a certain—variable—line on the Surface of the platforms. In order to obtain effective cooling over a maximum area of the platform surfaces, i.e. also in the area of the three-dimensional secondary flow, Specification DE 198 13 779 provides for a cooling-air ejection, termed ballistic cooling, in a direction corresponding to the radius, i.e. in a plane limited by the turbine axis and the radius, at a relatively steep ejection angle to the turbine axis with high impulse ratios, in which the cooling-air ejection holes forming at least one row are arranged in groups spaced from each other in turbine circumferential direction, each confined to an area from the leading edge to the pressure side of the respective nozzle guide vane. Accordingly, the intent of the so-called “ballistic cooling” in an area confined to the pressure side of the nozzle guide vanes is to bring the cooling medium to, and adequately cool also those platform surfaces, which are located in the area behind the three-dimensional inlet boundary layer separation line.
Specification EP 0 615 055 A1, whose technical teaching is also based on the above-mentioned principle of film cooling or ballistic cooling of the platforms, in contrast to the solution described in Specification DE 198 13 779 A1, provides for at least one circumferentially uninterrupted row of ejection ducts which, however, feature different diameters in the circumferential direction to obtain a certain mass flow distribution, enabling a maximum of full-surface cooling of the platform surface. Also with this cooling arrangement, the orientation of the ejection ducts, except for a certain incidence angle required for passing the platform or the combustion chamber wall, agrees with the plane established by the turbine axis and the radius.
However, the above cooling arrangements, due to a high degree of mixture with the hot-gas flow and an excessively large distance between the cooling air and the platform, are not capable of efficiently utilizing the blown-in cooling air and, moreover, ensuring an adequate degree of film cooling in all surface areas of the platforms, i.e. also in the downstream separation area of the boundary layer. In order to achieve an adequate degree of hot-gas shielding of the platforms, it will, therefore, be required to use a relatively high cooling-air proportion and/or provide a thermal barrier coating or enhance the effectivity of such a coating, with costs being increased correspondingly. In certain cases, a complex cooling system may be required for surfaces outside the hot-gas flow which would result in an increase of specific fuel consumption and costs, just as with the film cooling of the nozzle guide vane passage.